The inventors of the present application have successfully isolated a human plasma membrane-associated sialidase (NEU3) gene (see, Patent Document 1). Meanwhile, they have discovered that the expression of the gene is enhanced in various human cancers including human colon cancer (see, Non-Patent Document 1), prostate cancer, head and neck cancer and etc. with few exceptions. On the other hand, a transgenic mouse into which the gene was introduced were found to develop diabetes (see, Non-patent Document 2), and the polymorphism in the NEU3 gene in patients with diabetes was discovered and found to be deeply involved in onset of Type 2 diabetes.
As described above, it has been suggested that the enhancement of the expression level of the human plasma membrane-associated sialidase is deeply involved in canceration of cells and onset of diabetes. Based on this finding, the inventors of the present invention have provided a method of diagnosing cancer using an antibody that specifically recognizes a human plasma membrane-associated sialidase (see, Patent Document 2).
On the other hand, in recent years, as a method for specifically inhibiting the expression of a particular gene, a method using RNA interference (RNAi) caused by a double-stranded RNA having a sequence complementary to the gene has attracted attention. The mechanism of the method is considered as follows.
When a double-stranded RNA having a sequence complementary to a gene targeted for expression inhibition is incorporated into cells, the double-stranded RNA is cleaved with Dicer enzyme which belongs to the RNAase III family and processed into short double-stranded RNA fragments of about 21 to 23 nucleotides. The antisense strands of the RNA fragments are separately bound to proteins having ribonuclease activities to form complexes referred to as RISC(RNA-induced silencing complex). It is considered that when an antisense strand in RISC is bound to mRNA of a target gene, the mRNA is cleaved to inhibit the expression of the target gene (see, Non-patent Document 3).
It is generally known that, when a long-chain (of at least hundreds of nucleotides) double-stranded RNA is used for inhibiting the expression of a gene of a nematode, a fungus, Drosophila, a plant, or the like, a higher effect of inhibiting the expression can be achieved. However, in a mammal cell system, introduction of a long-chain double-stranded RNA activates an interferon signal pathway to cause cytotoxicity, thus it is considered that the method is difficult to be applied to mammal cells.
However, thereafter, a method of avoiding interferon responses by introducing a short double-stranded RNA (for example, of about 21 to 23 nucleotides) in advance has been developed, and became applicable to mammal cells (see, Non-Patent Document 4). Such a short double-stranded RNA is referred to as siRNA (small interfering RNA) and expected to be applied to a gene function analysis or a gene therapy using viruses (for example, see Non-Patent Document 5).
However, a siRNA that inhibits the expression of a human plasma membrane-associated sialidase (NEU3) gene has not been known, and a pharmaceutical composition for treating cancer or diabetes by using such a siRNA has not been known.
Patent Document 1: JP 3088681 B
Patent Document 2: JP 2003-55399 A
Non Patent Document 1: Proc. Natl. Acad. Sci., 99, 10718-10723, 2002
Non Patent Document 2: J. Biol. Chem., 278, 27896-27902, 2003
Non Patent Document 3: Molecular Medicine, 41(1), 10-29, 2004
Non Patent Document 4: Nature, 411, 494-498, 2001
Non Patent Document 5: Virus, 53(1), p 7-14, 2003